Problem: Which of these numbers is prime? ${21,\ 63,\ 73,\ 95,\ 98}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 21 are 1, 3, 7, and 21. The factors of 63 are 1, 3, 7, 9, 21, and 63. The factors of 73 are 1 and 73. The factors of 95 are 1, 5, 19, and 95. The factors of 98 are 1, 2, 7, 14, 49, and 98. Thus, 73 is a prime number.